All Things Digital

In anticipation of developing a quadcopter, I bought a Spektrum Dx5e/AR500 transmitter/receiver pair. Knowing that the quadcopter would probably take many months to design and build, my wife lovingly thought to buy me an RC Airplane to hold me over until I finished my quadcopter. She snooped on my computer and found that I had been looking at the Multiplex EasyStar.

The version I got (linked below) didn’t come with an Tx/Rx, servos, ESC, or battery. I jumped on HobbyKing.com and bought two servos, a 2-cell lipo battery, and a cheap brushed ESC. It turned out that the servos I bought were a low-voltage version and very weak. Because I didn’t want to wait for another shipment from China, I ordered some better servos from HorizonHobby.com.

My first flight went horribly! I was too anxious to fly and didn’t wait for a good day. Let’s just say I flew around for a few minutes with completely no control and then ended my performance with a full throttle nose dive into the ground. The cockpit area of the fuselage broke in half. To those of you reading this deciding whether to get into RC planes or not, don’t let this discourage you. I did EVERYTHING wrong. Not only was I too anxious, but I was also nervous. I left the throttle at full. This was a bad combination.

I fixed the fuselage using the same glue I put it together with and some clear box tape. It actually seemed to get even stronger! I waited for a calm day then attempted flight number two. I remembered to stay clam, set the throttle to full, hand launched, got control, then set the throttle to about half. I couldn’t believe how easy it was to fly! I was flying around with perfect control in no time. I even got brave and applied full throttle and pulled off a loop. It was ridiculously fun! Then I realized I needed to land sometime soon or it would land by itself (another nose dive into the ground?). Once I got close to the ground, I cut the throttle to about 1/4 power then slowly descended in a straight line. It smoothly slid across the grass until it stopped!

Aggressive Flight Modification:

After a few flights, I determined that the aircraft was a bit sluggish when looping and turning. I decided to do a few things to fix this:

Servo Cable Adjustment:
Each servo cable attaches to the corresponding flap using a threaded clamp that sits in one of three slots. When I first put the airplane together, I put the clamp in the least aggressive slot (furthest from the flap). I just moved all the clamps to the most aggressive location (closest to the flaps). This increased the aggressiveness of the aircraft dramatically.

Rudder Enhancement:
Unlike a typical RC airplane, the rudder on the EasyStar controls the tail and the roll of the aircraft. Its design is far too small for its purpose. I used some thin, stiff cardboard and super glue to extend the length of the rudder. The stock length is about 1″. I modified it to be about 6″. This turned out to be too much rudder when the aircraft was moving quickly. I cut the extended flap down to about 3.5″. It now works great. The plane now responds very fast but I also don’t get out of control when I am moving quickly through a turn.

Durability:

This airplane is super durable. I have crashed in one way or another numerous times! Most of the time nothing happens. I’m pretty sure my fuselage is now 40% glue, 45% tape, and 5% foam. It still flys like a champ! The location of the propeller is perfect for beginners. It has never been harmed in all my crashes.

Too Aggressive?:

Apparently my modification makes the aircraft more aggressive than the foam wings can handle. To get enough speed for the many tricks I’ve learned, I take the EasyStar up as high as I can while still feeling comfortable, then dive nose down. Sometimes I just do this to see how fast I can get going and how close I can get to the ground until I pull up. Each time I do this I can see the wings flexing back. One day my dives were getting really fast and one of the wings finally gave out. The wing snapped in half and the rest of the plane went spinning into the ground. I could probably beef up the wing strength with some carbon fiber rods, however, $25 for new wings isn’t too much to ask for 6 months of abuse.

The aim of this design is for an extremely aggressive quadcopter. My design goals are for the stabilization system to handle VERY abrupt changes in attitude and able to recover from any acrobatic mishap. I’ll be adding a mode for acrobatics where absolute angles are not used for stabilization. Instead, the stabilzer will hold to a rotational rate. If an acrobatic maneuver goes south, one switch flip will be able to bring the helicopter back to a stabile hover. The only remaining control not adjusted by the stabilizer is the throttle.

I’m sure that there will be additional items I will need but haven’t thought of. I will try to keep this list up to date for those of you using it for your own copter build.

I have been researching a variety of multi-rotor helicopter setups for some time. I’ve been trying to identify the strengths and weaknesses of each design type. I have come up with an initial design for my quad-rotor helicopter. In attempts to create a very aggressive aircraft, I have designed the propellers to be a close as possible, as large as possible, and the motors to have a high power to weight ratio. All of my assumptions about quadcopter design are very spectulative since I haven’t ever built one before.

My first matter of design is a powerful CPU. My fly-by-wire T-Rex 600 project used 4 Arduinos and they left a sour taste in my mouth. Arduinos are great for quick prototyping but anything with substance needs a better processor. Besides, I’m a Computer Engineer so I can’t justify using someone else’s poorly designed microcontroller libraries. My microcontroller of choice for this project is the LPC1768 ARM Cortex-M3 by NXP Semiconductors. It is a powerhouse! I’ve written most of the low-level hardware drivers and a few of the higher level routines, such as PID controllers and Collective Cyclic Throttle Mixing (CCTM). The combination of the ARM’s Cortex-M3 core with NXP’s hardware peripherials makes this as amazingly powerful design. ARM+NXP=Happiness!

For aircraft attitude measurement, I’m planning on using the 9DOF Sensor Stick from Sparkfun. Version 2 is still under design so I’ll have to wait on that. I’m going to implement 3 different types of sensor fusion algorithms and see which type works the best. The 3 algorithms are:

Complimentary Filter

Direction Cosine Matrix

Extended Kalman Filter

In the animation above, the two circles representing the propellers show the two sizes I will test. The inner circles are a 12×3.8″ APCprop and the outer circles are a 14×4.7″ APCprop. I haven’t seen another helicopter use the 14″ props before so I’ll get the 12″ props working first.

This is an explanation of the animation per level:

Level One (bottom):

LiPo battery

Receiver

Level Two (between the two metal plates):

4x Electronic Speed Controllers

Level Three (top):

CPU

3-Axis of Gyroscopes, Accelerometers, and Magnemeters

Outer Arms:

4x Motors

4x Props

I know that having the props close together makes the attitude harder to stabilize. On the other hand, having the props close together will (I hope) induce much more torque on the frame from the motors. This will help me overcome the lame yaw response of most quadrotor helicopters. I’m banking on the fact that my CPU will be running at 100 MHz and I’ll hopefully have the sensor fusion filters and PID controller running at 400+Hz. This should allow me to precisely adjust each axis of stabilization. For better discrete calculus computations (integration and differentation) I’ll try running the sensor fusion algorithms above 1kHz and only commanding the ESCs at their maximum speed (50Hz-400Hz). This will make the computations more accurate because each time step will produce less error.

About this blog

This blog is dedicated to the countless hours I spend tinkering and creating seemingly useless yet fun devices. I've spent numerous years improving my geekness and there is no end in sight. I like sharing my ideas and creations so feel free to ask :)